Reaction condition: 10 g solvent, 1.0 wt. % lignin, 0.1 mmol CuSO4, 1.0 MPa O2, 433 K, 4 h. Note: H, G and S represent the products from the structural units of the p -coumaryl alcohol (H), coniferyl alcohol (G), and sinapyl alcohol (S) in lignin respectively. a-without CuSO4.
Additionally, the structures of raw lignin and Re-lignins from different oxidation system were analyzed by FT-IR spectra (Figure 6). The absorbances at 3450 and 2912 cm-1 can be assigned to the vibration of O-H and C-H stretching respectively, and those at 1605, 1514, 1454 and 814 cm-1 are ascribed to the aromatic skeletal vibrations of lignin,42 which suggests that the skeletal structure of lignin is retained in Re-lignins. It is worth pointing out that the absorbance at 1740 and 1103 cm-1(characteristic absorption of C=O stretching and the H structural unit in lignin respectively) for Re-lignins from the microemulsion (point c in Figure 3) are weakened more significantly by comparison to that from other system, this observation further certifies that the developed microemulsion reactor is selective to tailor H unit in lignin.